Heat-sensitive recording methods have recently been increasingly used in the fields of facsimile and printers, due to advantages such as:
(1) no development step is required; PA0 (2) paper quality is approximately that of ordinary paper when paper is used as the support; PA0 (3) handling is easy; PA0 (4) high coloration density is obtained; PA0 (5) recording equipment therefor is simple and inexpensive; and PA0 (6) no noise is produced upon recording.
As heat-sensitive recording materials for such methoods, leuco coloration type heat-sensitive recording materials excellent in coloration density and coloration rate are predominantly used. However, leuco coloration type heat-sensitive recording materials have certain defects, in that (1) they can form color by handling after recording, (2) by heat, (3) by adhesion of a solvent to stain recording images, and (4) the formed color is removed by plasticizer contained in cellophane tapes.
In order to prevent coloration caused by careless handling, it has been known to add a particulate wax as disclosed in Japanese Patent Publication No. 14531/75 or to provide a coating layer for preventing permeation of the plasticizer as disclosed in Japanese Utility Model Application (OPI) No. 125354/81 (the term "OPI" as used herein refers to a "published unexamined Japanese Application"). However, these techniques are still unsatisfactory, and, where the possibility of falsification must be avoided, heat-sensitive recording materials have not been used. Thus, it has been eagerly desired to improve them. As a technique of preventing coloration in undesired portions after themal recording, it has been known to use a heat-sensitive recording material comprising a diazonium compound, a coupling component, an alkali-producing agent, or a coloration aid, and, after thermal recording, irradiate the material with light to decompose unreacted diazonium compound for the purpose of stopping further coloration, as disclosed in Japanese Patent Application (OPI) Nos. 123086/82, 125092/82 (corresponding to U.S. Pat. No. 4,411,979), U.S. Pat. No. 3,281,244, etc.
However, this recording material has the defect that it can undergo gradual pre-coupling during storage to produce unfavorable coloration (fog). Therefore, at least one of the color-forming components is allowed to exist in the form of non-continuous particles (i.e., a solid dispersion) to prevent the components from contacting with each other, thereby preventing precoupling. However, shelf storage properties of such recording material are still insufficient.
As another technique, it is known to separate the diazonium compound and the coupling component from each other by incorporating them in different layers for minimizing the contact therebetween. This technique improves shelf storage stability, but it concurrently so deteriorates thermal coloration properties that the material based on this technique fails to respond to high-speed recording with short pulse width.
Further, it is known to encapsulate either a coupling component or an alkali component with a non-polar wax substance (Japanese Patent Application (OPI) Nos. 4414/82 and 142636/82) or with a hydrophobic high molecular weight substance (Japanese Patent Application No. 192944/82) for separating it from the other component.
This encapsulating technique involves complicated steps of dissolving the wax or high molecular substance in a solvent, dissolving or dispersing the color-forming component in the resulting solution, then evaporating off the solvent to form microcapsules.
These techniques have attempted to overcome the fundamental fact that diazonium compounds possess a considerable solubility in water, but, in many cases, high-speed printability is sacrificed for the sake of improving shelf storage properties of heat-sensitive recording materials.